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The Lymphatic System.
The Lymphatic System is made up of a network of thin-walled vessels that carry a clear fluid. This system performs four major functions:

1. It collects excess water and proteins from the interstitial fluid that circulates around cells throughout the body, and returns them as lymph to the bloodstream.
2. It transports fats from the tissues surrounding thesmall intestine to the bloodstream.
3. It destroys micro organisms and other foreign substances.
4. It provides long-term protection against micro organisms and other foreign substances.

The lymphatic system begins with very small vessels called lymphatic capillaries, which are in direct contact with interstitial fluid. The system collects and drains most of the fluid that is forced out of thebloodstream, and accumulates in the spaces between cells. The small lymphatic capillaries merge to form larger lymphatic vessels, which pass through the lymph nodes. These larger vessels converge to form two main drainage ducts that return the excess fluid to the blood circulation, through the subclavian veins above the heart.
All the tissues of the body, except those of the central nervoussystem and the cornea, are drained by the lymphatic system. This system comprises aggregates of encapsulated lymphoid nodules called tonsils, the spleen, the thymus gland, and unencapsulated lymph nodules.
Lymphatic tissue contains specialized leukocytes called lymphocytes that perform the same protective functions as leukocytes in the blood. Most of the leukocytes essential to the function of lymphare produced in the bone marrow. The human body contains about 2 trillion lymphocytes. They are the backbone of the immune system and are the basis of the immune response.

Lymphatic capillaries and blood capillaries are similar in structure, but lymphatic capillaries are not part of a circuit of vessels as blood capillaries are. They are most abundant in the surfaces ofthe body, for example the dermis of the skin, and in the layers of the respiratory and digestive systems.
The lymphatic capillaries join with other capillaries to become larger collecting lymph vessels, that resemble veins, but their walls are thinner than venous walls, and they pass through specialized masses of tissue (the lymph nodes). They run parallel to blood vessels. Lymph vessels joinwith one another to form two large ducts: the Right Lymph duct and the Thoracic duct, that empty their contents into the subclavian veins
Lymph travels in only one direction (toward the subclavian veins), because of valves within the lymphatics that do not allow fluid to flow back. The lymphatic valves operate in the same way as the one-way valves in veins.

Scattered along thelymphatic vessels, like beads on a string, are small (1 to 25 mm in length) bean-shaped masses of tissue called lymph nodes. They are found mostly at the neck, armpit, thorax, abdomen, and groin. Most lymph passes through at least one lymph node on its way back to the bloodstream.
Lymph nodes filter out harmful micro organisms and other foreign substances from the lymph. They also initiate thespecific defence of the immune response. The outer portion of a node is the cortex, which contains lymphocytes. Lymph nodes produce about 10 billion lymphocytes every day.

There are three types of tonsils: 1) the single pharyngeal tonsil (adenoid) in the upper posterior wall of the nasopharynx, behind the nose; 2) the palatine tonsils, on each side of the soft palate, and 3) the lingualtonsils at the base of the tongue. Together, the tonsils form a band of lymphoid tissue that is strategically placed at the upper entrances to the digestive and respiratory systems, where foreign substances may enter easily.
Most infectious micro organisms are killed by lymphocytes at the surface of the pharynx, or are killed later, after the initial defences are set in motion by the tonsils....
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